The disclosed embodiments generally relate to micro-fabrication processes and more particularly to thin film liftoff techniques.
Liftoff is a method for patterning deposited films. A pattern is generally defined on a substrate using photoresist and standard photolithography. A film, usually metallic, is blanket-deposited all over the substrate, covering the photoresist and areas in which the photoresist has been cleared. During the actual lifting-off, the photoresist under the film is removed with solvent, taking the film with it, and leaving behind only the film which was deposited directly on the substrate.
FIGS. 1A-1C show an example of a conventional liftoff process. FIG. 1A shows a bilayer liftoff mask comprising a polymer layer 105 and a silicon dioxide layer 110, typically prepared by lithography and dry etching, applied to a substrate 115. FIG. 1B shows a thin film metal 120 applied by electron-beam evaporation. Thickness of the polymer layer 105 is generally greater than the thickness of the thin film metal layer 120, and precise and reproducible control of a liftoff mask thickness and undercut are difficult. Use of a sputter source is prohibited because sputtering may cause the sidewalls of the silicon dioxide and polymer layers to be coated creating spurious features. As shown in FIG. 1C, a solvent soak is used to lift off the mask, leaving the thin film 120 with substantially vertical sidewalls. Furthermore, sputter deposition techniques may conformally coat various surfaces of the mask, which may result in unwanted topographies at the edge of the film denoted herein as “flags.”
Some liftoff masks are made of material that might outgas when subjected to high vacuum. This may introduce impurities in the deposited metal film leading to degraded performance, for example, higher microwave losses or lower transition temperatures.
In some instances, dry, reactive ion or inert ion etching processes may cause damage to the underlying substrate, which can degrade the performance of a micro-fabricated transmission line for sub-mm radiation, because (1) if the dielectric thickness abutting the metal line is different across the length of the line, then there are unwanted reflections due to impedance changes across the length of the line, and (2) if the surface roughness of the dielectric increases; consequently, the dielectric loss due to excitations of two-level systems increases. As a result, the fabrication of low loss superconducting transmission lines is difficult, especially low loss interconnecting circuitry for the 0.3-3 THz spectral range.
FIGS. 2A and 2B illustrate some of the disadvantages of a dry etch process when used to fabricate superconducting structures. As shown in FIG. 2A, a structure includes a first layer of niobium 205, a layer of silicon 210, and a second layer of niobium 215. A layer of resist 220 is deposited over a portion of the second layer of niobium 215. The structure is then subjected to a dry etch process, however, as shown in FIG. 2B, the dry etch erodes the silicon layer producing a roughened layer that does not have a uniform thickness.
Some Si substrates consist of ultra-thin layers on the order of 0.5 microns under which a metallic layer is deposited. This metallic layer may act as a mirror, which results in optical interference during the photolithographic exposure process used to pattern the film. This interference may occasionally result in distortion of the patterned feature. This results in poor control of a liftoff mask pattern, especially with features in a two micron range.
Some techniques may use polymeric liftoff masks, however, polymeric liftoff masks may provide poor feature size control at a two micron scale. Also, substrate temperatures may exceed 120 degrees Celsius during metal deposition, causing some polymers to adhere to underlying layers and leading to poor liftoff.
It would be advantageous to provide micro-fabrication materials and techniques that overcome these and other disadvantages.